Excitation device and method for downhole seismic testing using the same

ABSTRACT

An excitation device for downhole seismic testing, includes an excitation hammer, a holding arm connected at one end thereof to the excitation hammer to allow the excitation hammer to rotate about the other end thereof, a support post having the other end of the holding arm rotatably coupled to an upper end thereof to support the excitation hammer and the holding arm at a certain height, and an excitation source configured to allow the support post to stand upright on the upper side thereof and which generates a seismic wave when struck by the rotated excitation hammer. The support post is formed at a lower end thereof with a securing section detachably inserted into a groove for the support post formed on the upper side of the excitation source such that the support post, the holding arm, the excitation hammer and the excitation source can be separated from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an excitation device and method fordownhole seismic testing using the same, and more particularly, to anexcitation device for downhole seismic testing, which is installed onthe ground to generate seismic waves with efficiency and accuracy andallows separation into individual units for convenient carriage andsecure installation, and a method for downhole seismic testing using thesame.

2. Description of the Related Art

For efficient management of water that is essential for human life andindustrial development, structures, such as dams, reservoirs, seawalls,banks, and the like, can be built not only with reinforced concrete, butalso in the form of an embankment structure through construction ofearth materials such as soil, sand, rock, and the like.

The crest of the embankment structure constructed of the earth materialsis generally paved with asphalt, concrete or other special materialhaving flexibility, or is protected by blocks having various shapes andsizes not only for the purpose of construction of a road or sightseeing,but also for the protection of an embanked body from various factors,such as erosion, weathering, water of infiltration, and the like. Sincethe embankment structure for water conservation or management is veryclosely related to variation of the underground water level in view ofstability and functions thereof, observation holes for observing theunderground water level are formed at major points on the crest of theembankment structure to allow periodic observation of variation in theunderground water level.

Large scale and deep excavation works are frequently performed in citiesfor foundation works for large buildings or structures or for works forconstruction of underground spaces, and various kinds of measurement areperiodically performed with respect to the surrounding ground duringexcavation works to determine stability of neighboring buildings. Inthis case, various observation holes for observing the underground waterlevel are formed corresponding to places on the ground near anexcavation site, the majority of the surface of which has been alreadypaved. Since the effect of excavation work on the surrounding ground canbe determined at various points through periodic measurement of thevariation of the underground water level, the measurement of thevariation of the underground water level is considered a crucial factor.

Generally, excavation work or the embankment structure for waterconservation or management is planned or designed in consideration ofthe aim or performance guarantee term in view of geotechnicalengineering or hydraulics. Even in this case, however, stability of atarget structure or the surrounding ground and structures can changeduring construction of the structure or over time due to abnormalvariation of material or design factors, unpredicted inner or outerfactors, and the like.

From this point of view, it is necessary to perform periodic andcontinuous measurement of the embankment structure or the conditions ofthe surrounding ground near an excavation site. Particularly, in theembankment structure for water conservation or management or the groundnear an excavation site in which the underground water level is higherthan an excavation base, there can be a gradual or rapid variation inengineering characteristics of the ground, which acts as an innerconstruction material, due to variation of the underground water levelor secondarily induced factors. Change in characteristics of the groundmaterial is directly related to local or overall stability of astructure. Particularly, dynamic stiffness at a small strain level, thatis, body wave velocity as a characteristic of quantitative seismic wavesis considered one of the most important material characteristics in therelated art.

Since behavior with respect to effective stress corresponding tobehavior of pore fluid is very important for ground materials, shearwave velocity (V_(S)) is considered a major factor in the body wavevelocity composed of compressional wave velocity (V_(P)) and shear wavevelocity (V_(S)). From this point of view, evaluation of target groundmaterials must be systemized based on a useful ground engineeringtechnique through periodic and continuous measurement of the shear wavevelocity.

Nevertheless, periodic and continuous measurement and evaluation of theshear wave velocity for most ground materials which require stabilityhas never been taken into consideration. Further, in evaluation ofoverall stability with respect to a target structure, non-periodicseismic testing under limited conditions, such as non-destructiveseismic testing on the ground surface, is generally used to confirmdistribution of the shear wave velocity in a material.

Generally, among the techniques for determining distribution of a shearwave velocity according to an increase in depth, a borehole seismic testmethod is performed through construction of boreholes and has higherreliability than a surface wave test method which is performed on theground surface. In particular, downhole seismic testing as illustratedin FIG. 1 has higher economic feasibility and efficiency than crossholeseismic testing and is thus actively applied to ground engineering.

Referring to FIG. 1, for downhole seismic testing, with a hexahedralexcitation source 10 such as a log placed on the ground surface, aborehole 40 is formed vertically from the ground surface according to anincrease in excavation depth and at least one receiver 50 for detectingexcavation is prepared. The downhole seismic test is performed in-situby obtaining a seismic wave signal generated on the ground surface whilechanging the location of the receiver 50 according to variation inexcavation depth. Here, when confirming excitation from an initialmotion detector 20 connected to the excitation source 10, a dynamicsignal detector 30 placed on the ground obtains the seismic wave signalfrom the receiver 50 inside the borehole 40.

Seismic wave signals according to depth as obtained in-situ are analyzedthrough several steps to determine distribution of the shear wavevelocity according to excavation depth (Chang-guk Sun, Hong Jong Kim,Jong-hong Jung, Gyung-ja Jung, 2006, “Synthetic Application of SeismicPiezo-cone Penetration Testing for Evaluating Shear Wave Velocity inKorean Soil”, Geophysical exploration, Volume 9, No. 3, pp. 207-224.).Then, variation in conditions and stability of a target structure can beevaluated based on quantitative variation according to temporal andspatial differences of the shear wave velocity.

SUMMARY OF THE INVENTION

The present invention is directed to providing an excitation device fordownhole seismic testing, and a method for downhole seismic testingusing the same, which can be conveniently disassembled and assembled toallow convenient carriage.

The present invention is also directed to providing an excitation devicefor downhole seismic testing and a method for downhole seismic testingusing the same, which can be secured to target ground for periodic andcontinuous evaluation of conditions of the target ground while allowingthe target ground to perform normal functions. Here, the excitationdevice is configured to secure an absolute location of an excitationsource at a certain location on the ground by providing a referencesocket within the target ground.

The present invention is also directed to providing an excitation devicefor downhole seismic testing and a method for downhole seismic testingusing the same, which may maximize expressed energy while securingorientation of seismic waves upon generation of the seismic waves. Here,the present invention is directed to providing test data for multiplepurposes through diversification of the intensity of the expressedenergy.

The present invention is also directed to providing an excitation devicefor downhole seismic testing and a method for downhole seismic testingusing the same, which employs an existing observation hole for observingan underground water level, instead of constructing a separate test holewhich requires an operation for abandoning the hole, and permitscombinational analysis of quantitative distribution variation of shearwave velocity and compressional wave velocity corresponding to variationof an underground water level at the same spatial location, therebysignificantly improving reliability of stability testing of targetground.

In accordance with one aspect, an excitation device includes anexcitation hammer; a holding arm connected at one end thereof to theexcitation hammer to allow the excitation hammer to rotate about theother end thereof; a support post having the other end of the holdingarm rotatably coupled to an upper end thereof to support the excitationhammer and the holding arm at a certain height; and an excitation sourcewhich is configured to allow the support post to stand upright on theupper side thereof and generates a seismic wave when struck by therotated excitation hammer. Here, the support post is formed at a lowerend thereof with a securing section detachably inserted into a groovefor the support post formed on the upper side of the excitation sourcesuch that the support post, the holding arm, the excitation hammer andthe excitation source can be separated from each other.

The excitation source may be formed at a lower side thereof with anindentation frame into which a transfer wedge is removably inserted, andthe transfer wedge may include a head inserted into the indentationframe and a tip directly inserted into target ground or into a wedgereference socket in the target ground.

The support post may have an “L” shape to secure a rotation space forallowing the excitation hammer to strike a side surface of theexcitation source.

The excitation source may be made of wood.

The indentation frame and the transfer wedge may be made of aniron-based material.

The excitation source may have a parallelepiped shape having a width of30 to 50 cm, a height of 20 to 40 cm, and a length of 50 to 80 cm.

The groove for the support post may have a depth of one third the heightof the excitation source.

The indentation frame may be formed at a place a quarter of the lengthof the excitation source from a distal end of the excitation source andhave a depth of one third the height of the excitation source.

In accordance with another aspect, a seismic test method includes:removing an upper soil and a protective layer from target ground tosecure an installation space; placing the excitation device having anyone of the aforementioned features inside the installation space;placing a detection receiver in an observation hole for observing anunderground water level, and obtaining a seismic wave signal generatedfrom the excitation device to perform downhole seismic testing; andremoving the excitation device and inserting a cover block into theinstallation space, after finishing the test.

The protective layer may include a ground pavement material.

The installation space may be formed orthogonal to the observation hole.

A boundary block made of a stretchable material may be disposed on avertical plane of the installation space.

The cover block may include a gripper on an upper surface thereof

The target ground in the installation space may be formed with a wedgereference socket.

The wedge reference socket may be made of wood.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the inventionwill become apparent with reference to the following exemplaryembodiments in conjunction with the accompanying drawings, in which;

FIG. 1 is a schematic illustration of a conventional downhole seismictest method;

FIG. 2 is a schematic illustration of an excitation device according toone exemplary embodiment of the present invention;

FIG. 3 is a schematic illustration of a cover block of the excitationdevice according to the exemplary embodiment of the present invention;and

FIG. 4 is a schematic illustration of a downhole seismic test methodaccording to one exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will now be described indetail with reference to the accompanying drawings.

For most embankment structures, a surface layer of the crest issubjected to pavement for preventing constituent materials fromsuffering erosion or weathering, or to block construction for protectionor performance guarantee. For the ground around an excavation site, thesurface layer is also subjected to pavement or other processes usingpavement blocks. For such reasons, target ground has a sub-base, a base,and surface layers, such as ascon pavement, flexible pavement, pavementblocks, and the like, formed thereon. To provide continuous stabilityevaluation with respect to target facilities and surround groundincluding ground materials, which are covered with additional materialsincluding pavement or processing blocks, an observation hole forobserving an underground water level is formed to a predetermined depthfrom the ground surface at an essential place in terms of engineeringfeasibility to enable periodic measurement of the underground waterlevel.

In the present invention, an excitation device for downhole seismictesting according to one exemplary embodiment of the invention isprovided to target ground through upper layers (surface layer, base, andsub-base), which are formed on the ground material for protection of theground material or other purposes.

FIG. 2 is a view of an excitation device according to one exemplaryembodiment of the present invention.

Referring to FIG. 2, the excitation device according to the embodimentincludes an excitation hammer 110, a holding arm 120 which providesrotational force to the excitation hammer 110, a support post 130 whichsupports the excitation hammer 110 and the holding arm 120 at apredetermined height, an excitation source 140 which generates a seismicwave when struck by the excitation hammer 110, and a transfer wedge 150inserted at one side thereof into the excitation source 140 and at theother side into the ground to transfer the seismic wave into the ground.

In FIG. 2, (a) is a side view illustrating constitution and operation ofthe excitation hammer 110, the holding arm 120 and the support post 130,(b) is a plan view of the excitation source 140, (c) is a side-sectionalview illustrating the excitation source 140 and the transfer wedge 150,and (d) is a bottom view of the excitation source 140 and the transferwedge 150.

Referring to FIG. 2( a), the support post 130 is provided at a lower endthereof with a securing section 131, which may be inserted into andsecured to a groove for the support post 141 of the excitation source140, and has an upper end to which one side of the holding arm 120 issecured.

The support post 130 serves to maintain the holding arm 120 and theexcitation hammer 110 connected to the holding arm 120 at apredetermined height while securing a space for rotation of theexcitation hammer 110. Specifically, as shown in FIG. 2( a), the supportpost 130 may generally have an “L” shape. The support post is secured atthe upper end thereof to one side of the holding arm 120 and is formedat the lower end thereof with the securing section 131, which may beinserted into and secured to the groove for the support post 141 of theexcitation source 140. The securing section 131 may be easily insertedinto or removed from the groove for the support post 141, so that theexcitation source 140 and the support post 130 can be easily separatedfrom each other, thereby allowing convenient carriage of the supportpost 130, the holding arm 120 and the excitation hammer 110.

As described above, the holding arm 120 is secured at one end thereof tothe upper end of the support post 130 and is connected at the other endthereof to the excitation hammer 110. Here, the other end of the holdingarm acts as a free end. Thus, the one side of the holding arm 120coupled to the support post 130 acts as a rotational axis and the otherside of the holding arm 120 freely rotates about the rotational axis.

The one side of the holding arm 120 may have a hinge structure to act asa rotational axis, and be optionally provided with a separate goniometerwhich can measure the rotational angle of the holding arm 120, that is,the rotational angle of the excitation hammer 110. The rotational angleof the holding arm 120 may be in the range of 1 to 180 degrees.

The excitation hammer 110 is secured at one side thereof to the free endof the holding arm 120 such that the other side of the excitation hammer110, that is, the head of the excitation hammer, can be rotated byrotation of the holding arm 120.

The excitation hammer 110, the holding arm 120 and the support post 130may be constituted as a pair of excitation hammers, a pair of holdingarms, and a pair of support posts, respectively.

Next, referring to FIGS. 2( a), (b) and (c), the excitation source 140according to this embodiment generally has a parallelepiped shape.Advantageously, the excitation source 140 may have a width of 30 to 50cm, a height of 20 to 40 cm, and a length of 50 to 80 cm in terms oftest performance, convenient carriage, and manufacturing efficiency.

In some embodiments, the excitation source 140 may be made of a log.Referring to FIGS. 2( a) and 2(b), the excitation source 140 is formedat either side thereof with the groove for the support post 141, intowhich the securing section 131 of the support post 130 can be securelyinserted. In some embodiments, the groove for the support post 141 mayhave a depth of one third the height of the excitation source 140.

The grooves for the support post 141, which are formed at opposite sidesof the excitation source 140 in the longitudinal direction of theexcitation source, are normally filled with the same wood material asthat of the support post 141, and the excitation hammer 110, the holdingarm 120 and the support post 130 are kept separate.

In downhole seismic testing, both side surfaces of the excitation source140 are struck in a lateral direction, or the upper surface of theexcitation source 140 is struck in a vertical direction to generateseismic waves, which mainly consist of a shear wave or a primary wave.When the support post 130 is not installed, it is possible to obtainexcitation through artificial striking only with the excitation hammer110 in the lateral direction and exhibition of the shear wave causedthereby.

Generally, artificial striking may cause differences in magnitude ofexcitation energy upon each strike, which limits quantitative comparisonanalysis based on differences in amplitude of the seismic wavesaccording to location. Thus, in order to overcome such a limit, thesupport post 130 is connected at the upper end thereof to the excitationhammer 110, and the holding arm 120, which permits initial positioningbefore striking operation of the excitation hammer 110, is provided toan upper portion of the support post 130 connected to the excitationhammer 110 to guarantee that the excitation hammer strikes theexcitation source with the same energy each time. In some embodiments, amulti-stage holding arm, which can be set at various angles, may be usedin consideration of diversification of excitation energy.

Further, referring to FIGS. 2( b) and (c), the excitation source 140 isprovided at opposite lower sides with indentation frames 142, into whichthe transfer wedges 150 can be removably inserted. In some embodiments,each of the indentation frames 142 is formed at a place a quarter thelength of the excitation source 140 from a distal end of the excitationsource and has a depth of one third the height of the excitation source140.

The indentation frames 142 are provided to the lower side of theexcitation source 140 made of wood, and may be made of an iron-basedmaterial. The indentation frames 142 serve to receive the transferwedges 150, each of which has a large chisel shape. In some embodiments,the transfer wedges 150 may also be made of an iron-based material.

More specifically, a thick head of each transfer wedge 150 is insertedinto the corresponding indentation frame 142 in the excitation source140, and a sharp tip of the transfer wedge 150 is directly inserted intotarget ground or into a receiving section formed in the target ground.Since the transfer wedges 150, which are placed inside the excitationsource 140 and extend to the target ground, are key to any deformationbehavior with respect to the target ground upon excitation duringdownhole seismic testing, the transfer wedges 150 serve not only tosecure orientation of the seismic waves, but also to maximize expressedenergy.

Herein, for periodic downhole seismic test and continuous evaluationregarding conditions of excavated ground and a structure for waterconservation or management, which has a crest acting as a general roadand is constructed on target ground having upper protective layers(surface layer, base, sub-base), a cover block 200, which permitsefficient positioning of the excitation device and serves as a normalroad in normal times, and a method for application thereof aredisclosed.

FIG. 3 illustrates a cover block of the excitation device according tothe exemplary embodiment of the present invention.

Specifically, FIG. 3( a) is a side sectional view illustrating aninitial structure of a ground surface paving material and an excavationtarget area for installation of the excitation device, FIG. 3( b) is aside sectional view illustrating a cover block 200 applied to theexcavation target area, and FIG. 3( c) is a side sectional view of theexcitation device after opening the cover block 200.

In FIG. 3( a), parts of protective layers (surface layer, base,sub-base) and target ground are excavated to provide a space forinstallation or removal of the excitation device.

First, the space for installation or removal of the excitation deviceand geometric features of the cover block 200 will be described. Forapplication of the cover block 200, the space is defined to sufficientlyenclose the excitation device for downhole seismic testing. Inparticular, the space may have a sufficient length in the longitudinaldirection so as not to obstruct free rotational striking of theexcitation hammer 110 during downhole seismic testing and has a widthabout 10 cm greater from both boundaries than the width of theexcitation source.

Further, excavation is performed to a depth to remove the upper layersof the target ground and a very minute upper portion of the targetground. Further, as shown in FIG. 3( b), the cover block 200 is made ofa lightweight material in order to permit efficient attachment andremoval thereof and is provided on an upper surface thereof with agripper 210, which is normally depressed coplanar with the upper surfaceof the cover block and is then pulled above the upper surface uponelevation of the cover block.

After excavation, boundary blocks 220 made of a stretchable materialsuch as rubber are provided to four vertical planes of the space andserve as stretchable linkages between the cover block 200 and the upperconstruction layers on the target ground. Further, as shown in FIGS. 3(b) and (c), wedge reference sockets 300 are formed in the target groundcorresponding to the shape and position of the transfer wedges 150 whichare used together with the excitation source 140. In some embodiments,the wedge reference sockets 300 may be made of wood when taking intoconsideration rigidity, flexibility, and plasticity of iron and soil. Byinserting such wooden wedge reference sockets 300 into the targetground, it is possible to guarantee that the excitation source ispositioned at the same place each time downhole seismic testing isperformed.

In some embodiments, a space to be equipped with the cover block 200 isformed by excavating the protective layers (surface layer, base andsub-base) and the target ground (see FIG. 3( a)). Then, with the wedgereference sockets 300 secured under the space, the cover block 200covers the space to allow the excavated part of the target ground to beused in normal times (see FIG. 3( a)). Then, during testing, the coverblock 200 is removed therefrom to place the excitation device on thetarget ground for downhole seismic testing.

Next, a method for downhole seismic testing according to one exemplaryembodiment of the invention using an existing observation hole forobserving an underground water level will be described with reference toFIG. 4.

FIG. 4 illustrates a downhole seismic test method according to oneexemplary embodiment of the present invention

In FIG. 4, an observation hole H for observing an underground waterlevel to be equipped with an excitation device for downhole seismic testand a test receiver R is shown.

In FIG. 4, (a) is a top view of the excitation device and theobservation hole H, illustrating conditions for downhole seismictesting, and (b) is a side sectional view of the excitation device andthe observation hole H, illustrating the conditions for downhole seismictesting.

The observation hole H generally has an inner diameter of about 50 mm,which is smaller than a conventional test hole used for downhole seismictesting and having an inner diameter of about 60 to 70 mm. Thus, to usethe observation hole H as the test hole for downhole seismic testing, asmall detection receiver R having a diameter of about 45 mm is usedinstead of a general receiver R having a diameter of about 60 mm. Such asmall detection receiver R may be conveniently manufactured throughapplication of three commercially available speedometers in threedirections. Alternatively, any small, commercially available receiver(for example, Geostuff, 2010, Geostuff Wall-Lock Borehole Geophones,http://www.geostuff.com/geophone.htm, Accessed 11 November 2010.) mayalso be used.

In addition, the cover block 200 replacing the ground surface pavementfor installation of the excitation source for downhole seismic testingis positioned orthogonal to the observation hole H for observing anunderground water level (that is, such that the excavated installationspace becomes orthogonal to the observation hole) while being separatedabout 2 m to 3 m therefrom, thereby eliminating a need for a test holefor downhole seismic testing through additional excavation whileguaranteeing multipurpose use of the observation hole H. Furthermore,since the observation hole H is used for observation of the undergroundwater level in normal times, there is no need to abandon the observationhole after seismic testing. In addition, combinational analysis ofquantitative distribution variation of shear wave velocity andcompressional wave velocity may be performed corresponding to variationof the underground water level at the same location, therebysignificantly improving reliability of stability testing for the targetground.

According to the exemplary embodiments of the invention, the excitationdevice may be conveniently disassembled and assembled, thereby enablingconvenient carriage thereof.

In addition, the excitation device may be secured to target ground toperform periodic and continuous evaluation of conditions of the targetground while allowing the target ground to perform normal functions. Inparticular, the excitation device may secure an absolute location of anexcitation source on the ground by providing a reference groove withinthe target ground, thereby improving reliability of test data.

Further, the excitation device may maximize expressed energy whilesecuring orientation of seismic waves and may provide test data formultiple purposes through diversification of the intensity of theexpressed energy.

Further, the excitation device employs an existing observation hole forobserving an underground water level, instead of forming a separate testhole which requires an operation for abandoning the hole, and permitscombinational analysis of quantitative distribution variation of shearwave velocity and compressional wave velocity corresponding to variationof an underground water level at the same location, therebysignificantly improving reliability of stability testing with respect totarget ground.

Some exemplary embodiments have been disclosed in the specification anddrawings. It should be understood that the terms used in theseembodiments are provided for the purpose of illustration and are notintended to limit the scope of the invention set forth in theaccompanying claims. Therefore, it will be apparent to those skilled inthe art that various modifications, changes, alterations, and equivalentembodiments can be made without departing from the spirit and scope ofthe invention. The scope of the invention should be limited only by theaccompanying claims and equivalents thereof

1-8. (canceled)
 9. A seismic test method includes: removing an uppersoil and a protective layer from a target ground to secure aninstallation space placing an excitation device inside the installationspace, the excitation device comprising; an excitation hammer; a holdingarm connected at one end thereof to the excitation hammer to allow theexcitation hammer to rotate about the other end thereof; a support posthaving the other end of the holding arm rotatably coupled to an upperend thereof to support the excitation hammer and the holding arm at acertain height, the support post being formed at a lower end thereofwith a securing section detachably inserted into a groove for thesupport post formed on an upper side of an excitation source such thatthe support post, the holding arm, the excitation hammer and theexcitation source can be separated from each other; and an excitationsource which is configured to allow the support post to stand upright onthe upper side thereof and generates a seismic wave when struck by therotated excitation hammer; placing a detection receiver in anobservation hole for observing an underground water level, and obtaininga seismic wave signal generated from the excitation device to performdownhole seismic testing; and removing the excitation device andinserting a cover block into the installation space, after finishingtesting.
 10. The method of claim 9, wherein the protective layercomprises a ground pavement material.
 11. The method of claim 9, whereinthe installation space is formed orthogonal to the observation hole. 12.The method of claim 9, wherein a boundary block made of a stretchablematerial is disposed on a vertical plane of the installation space. 13.The method of claim 9, wherein the cover block comprises a gripper on anupper surface thereof
 14. The method of claim 9, wherein the targetground in the installation space is formed with a wedge referencesocket.
 15. The method of claim 14, wherein the wedge reference socketis made of wood.
 16. The method of claim 9, wherein the excitationsource is formed at a lower side thereof with an indentation frame intowhich a transfer wedge is removably inserted, and the transfer wedgecomprises a head inserted into the indentation frame and a tip directlyinserted into the target ground or into a wedge reference socket in thetarget ground.
 17. The method of claim 9, wherein the support post hasan “L” shape to secure a rotation space for allowing the excitationhammer to strike a side surface of the excitation source.
 18. The methodof claim 9, wherein the excitation source is made of wood.
 19. Themethod of claim 9, wherein the excitation source has a parallelepipedshape having a width of 30 to 50 cm, a height of 20 to 40 cm, and alength of 50 to 80 cm.
 20. The method of claim 9, wherein the groove forthe support post has a depth of one third the height of the excitationsource.
 21. The method of claim 16, wherein the indentation frame andthe transfer wedge are made of an iron-based material.
 22. The method ofclaim 16, wherein the indentation frame is formed at a place a quarterof the length of the excitation source from a distal end of theexcitation source and has a depth of one third the height of theexcitation source.